In order to make a surface of a glass substrate water-repellant, a water repellant film such as a fluorine-based film is formed on the surface. A water repellant glass plate having a water repellant film is used for a window of vehicles or buildings. In such a window, the glass plate as a substrate is usually composed of a glass composition such as a soda-lime silicate including an alkali metal.
Japanese Laid-open Patent Publication No. 60-40254 discloses a water repellant glass plate as described above which includes a primer film having a thickness of 0.1 .mu.m or more formed by applying a silane compound such as a silane coupling agent to a surface of a glass plate, and a thin film of a compound having a polyfluoroalkyl group on the primer film. The organic polymer chains in the primer layer, which is thicker than usual, are entangled so that the hardness of the water repellant film is kept to some extent.
Japanese Laid-open Patent Publication No. 4-338137 discloses a water repellant glass plate having a SiO.sub.2 -based monolayer film, wherein non-metal atoms in the film are partially replaced with fluoroalkyl groups. The film can be produced by a sol-gel method in which fluoroalkylsilane is one of starting materials. A described example of the thickness of the monolayer film is 0.18 .mu.m.
Japanese Laid-open Patent Publication No. 4-239633 discloses a water and oil repellant film that has a substrate such as a glass plate, a first layer having an uneven surface formed from a mixture of silicate glass and fine particles on the glass plate, and a second monomolecular layer including a fluorocarbon group on the first layer. The first layer has an unevenness of about 10 .mu.m, which can improve the repellant effect of the second layer having a thickness between 1 .mu.m and 5 .mu.m.
Japanese Laid-open Patent Publication No. 5-238781 discloses a water repellant glass article that has a primer layer of silicon oxide and a water repellant layer formed from an agent including a perfluoroalkyl group. Although the ways of forming the silicon oxide primer layer such as a pyrolyzing method, a sol-gel method and a sputtering in the atmosphere including oxygen are disclosed, there is not described what is a preferable state of the primer film from a bond-structure point of view.
Japanese Laid-open Patent Publication No. 4-132637 discloses a contamination resistant glass plate having a monomolecular film composed of surface-active agents including a fluorocarbon group that shows water and oil repellancy. This monomolecular film can be formed by chemically binding a silane-based surface-active agent including a fluorocarbon group directly to the surface of the glass plate via the oxygen or nitrogen atoms. Although this art may be applied to the surface of a protective layer on a glass plate, there is not specifically described what is a preferable protective layer.
The water repellant glass plates as described above cannot provide a long-term durability for the water repellant effect that is sufficient to meet the requirement when the glass plate is used for vehicles or buildings.